The present invention relates to a semiconductor device for converting light and sound to electric signals.
Diodes, lateral bipolar transistors, and MOS transistors are known as a photoreceptor device incorporated in a semiconductor integrated circuit.
FIG. 1 is a sectional view showing a photoreceptor device having a conventional MOS transistor structure.
In FIG. 1, reference numeral 1 denotes a p-type silicon substrate; 2, an n.sup.+ -type source region; 3, an n.sup.+ -type drain region; 4, an n.sup.+ -type polysilicon gate electrode; 5, a gate oxide film; 6, an SiO.sub.2 film; 7, an interconnection; 8, an Al light-reflecting film; and 9, light.
The source region 2, the drain region 3, the gate electrode 4, and the gate oxide film 5 constitute a MOS transistor. In the MOS-FET having this structure, a photoelectrically converted signal is detected by changing the current flowing between the drain and source regions.
Since the light 9 is incident on the photoreceptor portion from above the chip, no multilevel interconnection that interrupts the light 9 can be formed. For this reason, the design for a large-scale Integration (LSI) incorporating many photoreceptor devices such as image sensors is greatly limited, and the chip size cannot be decreased.
In recent LSIs, the number of interconnections is increasing to four or five. When a microprocessor, a memory, a logic, and an image sensor are mounted on one chip, the whole chip design is greatly limited because no multilevel interconnection can be formed at the image sensor portion.
Since the light 9 is incident on the upper surface of the silicon substrate from the electrode interconnection side, the light 9 is absorbed and reflected by the electrode interconnection and the gate, resulting in a low photoelectric conversion efficiency.
FIG. 2 is a view showing the layout of a conventional LSI.
In FIG. 2, reference numeral 11 denotes a microprocessor; 12, a logic and a memory; and 13, an image sensor. With an increase in integration degree, a multilevel interconnection made up of three or more layers is desirably formed on the image sensor 13. However, no multilevel interconnection can be formed on the image sensor 13 because light is incident on the silicon substrate from the multilevel interconnection side in the conventional photoreceptor device structure.
When an LSI incorporating a photosensor and an image sensor with a multilayered structure is to be mounted on a logic LSI, a memory LSI, or the like, the upper LSI faces down and is connected to the lower one. Therefore, the conventional LSI incorporating a photosensor and an image sensor cannot be mounted with such a multilayered structure.
FIG. 3 is a view showing a typical CCD as a conventional image sensing device for photographing images.
FIG. 3 shows the state upon applying a clock 3. A potential well 22 is formed in that portion of a silicon substrate 21 facing a gate G of a clock 3, and charges 23 are generated in accordance with the intensity of incident light. An optical signal is converted into an electric signal as a charge amount, and the electric signal is stored in the potential well 22.
After the signal is stored, clocks are cyclicly applied from 3.fwdarw.1.fwdarw.2 to transfer the electric signal rightward in FIG. 3 and output it from the final output stage.
In the image sensing device, since the signal enters the chip from above it, the signal is reflected and absorbed by the gate electrode material, resulting in a low photoelectric conversion efficiency.